When I go to the doctor even to get a shot, they tape a little gauze over where they stuck the needle
The medics glued my recent pacemaker replacement incision. Never had a bandage on it. Zimmerman apparently has a head of hair. The area would have to be shaved to apply any kind of bandage. I doubt that would happen in the back seat of a police vehicle in the dark. Any treatment would likely have been a little bit of a cleanup.